This invention relates to improvements in a noise suppression circuit or a muting circuit used in an FM receiver of the type utilizing a ratio detector as a frequency discriminator.
In FM receivers, it is generally well known that noise is extremely low when the received radio wave has a sufficiently high intensity but in the absence of a received radio wave or when it is very weak, the resulting audio sound is distorted. Noise is also increased in the region in which the amplitude limiter or the ratio detector disposed in the FM receiver does not fully perform the amplitude limiting function. Therefore it is desirable to provide FM receivers with a noise suppression circuit for automatically suppressing noise under the abovementioned circumstances.
To this end, there have already been proposed a variety of so-called squelch circuits. Simple squelch circuits can be divided into two types, a mechanical type and an electronic type. In the mechanical type of squelch circuit, the intensity of the particular received radio wave is detected and a relay is operated in response to the detected intensity of the radio wave. Then, if necessary, the associated audio frequency output circuit is short circuited to prevent the application of noise to the loudspeaker or the like used for auido output. In the electronic type of squelch circuit, the noise component is taken from the frequency discriminator circuit only and amplified to provide corresponding a DC component. This DC component is applied to a squelch control circuit to change the bias applied to an associated audio frequency amplifier circuit to suppress the operation thereof.
These two types of squelch circuits can be complicated in circuit configuration and therefore expensive. Thus it has been difficult to use them with other than the high grade FM receivers. Further such squelch circuits have problems in that uncomfortable clicks are apt to be generated because their operation is too sensitive and the audio is interrupted suddenly.
Accordingly it is an object of the present invention to provide a new and improved noise suppression circuit for use having an FM receiver for suppressing noise with a simple, inexpensive circuit configuration and preventing uncomfortable clicks from being generated during the suppression of noise even when employed where the electric field intensity violently changes such as on FM receivers installed in motor vehicles.